Light straight run naphtha, virgin naphtha and other refinery streams containing C.sub.5 and C.sub.6 paraffins, are useful sources of blending stock for gasoline pools, but, under current market conditions and ecological considerations, it is necessary to raise the Research Octane Number (RON) of the stock to about 85 to 90 (RON), without the addition of alkyl lead compounds. Since the phase-out of lead compounds began, refiners have relied on isomerization and reforming, e.g., platforming, steps to increase the octane to above about 80 RON and further, have integrated separation processes to separate the isomerate or reformate into higher and lower octane fractions, with the higher octane fraction typically about 90 RON, being sent to the gasoline pool. The lower octane fraction(s) may be recycled to the isomerization step for further conversion and higher yields of multi-branched, high octane paraffins.
In one approach to the upgrading of isomerate streams, U.S. Pat. Nos. 4,717,784 and 4,855,529 disclose similar processes for isomerizing a hydrocarbon feed and separating the dimethyl branched paraffins produced in the reaction from the n-paraffins and monomethyl branched paraffins produced by utilizing a size- or shape-selective adsorbent. In the adsorptive separation step, the straight chain and singly branched paraffins are adsorbed into the pores of the molecular sieve while the more highly branched paraffins are excluded from entering the pores. The selectively adsorbed straight chain paraffins and/or the singly branched paraffins are recycled to the isomerization zone for further conversion to more highly branched paraffins. In this separation, the useful adsorbents have pore sizes in the range from 4.5.times.4.5 .ANG. to 5.5.times.5.5 .ANG. which exclude the di-branched paraffins from the pores of the molecular sieve. The preferred desorbent in the processes is hydrogen. The patentees teach that only sieves within this range are operative and others having pore sizes outside the range are too small or too large to perform the separation.
U.S. Pat. No. 2,956,089 discloses a process for separating di-branched and cyclic hydrocarbons from singly-branched hydrocarbons using 13A zeolitic molecular sieves. The single-branched hydrocarbons are selectively adsorbed and recovered by desorption. The disclosed feed did not contain isopentane.
U.S. Pat. No. 2,935,539 discloses a process for extracting di-branched paraffins from less highly branched chain paraffins with metallic amine complex-exchanged aluminosilicate zeolites. Desorption may be effected by, e.g., heating the adsorbent or displacement by a hydrocarbon boiling higher or lower than the extract material.
Neuzil U.S. Pat. No. 3,706,813 discloses the separation of dimethyl butanes from methyl pentanes and normal hexanes by selectively adsorbing the multi-branched components on barium- or barium and potassium-exchanged X or Y zeolites containing from 1-10% (wt.) water. Neuzil additionally discloses that the selectivity of the above adsorbents is reversed when the adsorbent is dry, presumably below about 1% (LOI), namely, that the normal and singly-branched paraffins are selectively adsorbed. Neuzil disclosed light desorbents, e.g., isobutane and isopentane, although other multi-branched, singly-branched, normal or cyclic paraffins having at least one carbon number less or greater than the feed components were stated to be useful.
Isomerization processes for converting normal paraffins to mono- and di- or more highly branched chain paraffins are commonly used as a method for increasing the octane rating of refinery streams containing normal paraffins. Illustrative of the isomerization process are U.S. Pat. Nos. 3,755,144 to Asselin and 2,966,528 to Haensel.
U.S. Pat. No. 3,755,144 also discloses the separation of normal paraffins from the isomerization reactor effluent using a molecular sieve in a simulated moving bed system and recycling the normal paraffins to the isomerization reactor after recovering the desorbent. Isohexane is recovered from the extract in a deisohexanizer column and recycled to the isomerization reactor; isopentane and dimethyl butane are taken overhead while cyclic paraffins are recovered in the bottoms of the deisohexanizer. U.S. Pat. No. 2,966,528 further discloses a swing bed system operating under liquid phase conditions for the separation of normal hydrocarbons from branched hydrocarbons in which the normal hydrocarbons are selectively adsorbed and desorbed from the adsorbent with a normal paraffin desorbent.
Accordingly, it is an object of the invention to provide a process for continuously separating di-branched paraffins from normal paraffins and mono-branched paraffins produced by an isomerization reaction with a particular adsorbent/desorbent combination by which the mono-branched, cyclic and normal paraffins having lower octane numbers are selectively adsorbed onto the desorbent. High octane components, di-branched paraffins and isopentane, are relatively selectively non-adsorbed by the adsorbent and are recovered from the non-selective void volume as raffinate. Through the use of an X zeolite molecular sieve having at least two cations from Group IIA metals at the exchange sites, applicants achieve excellent separation of multi-branched paraffins from lower octane mono-branched paraffins and normal paraffins. In addition, isopentane, a mono-branched paraffin which has a high octane rating, i.e., about 92, is also selectively non-adsorbed and separate recovery of isopentane for enhancing the octane rating of gasoline blends is avoided. The lower octane mono-branched paraffins (other than isopentane), cyclic paraffins and normal paraffins are desorbed from the selective pore volume by a carefully selected desorbent and recycled to the isomerization step for further upgrading. The desorbent is a branched or cyclic paraffin having a boiling point different from the feed composition.